The 9th Circuit U.S. Court of Appeals opinion issued late Monday is also unusual for ordering the transfer of a lower-court judge from the case, finding that U.S. District Judge Richard A. Jones made "numerous, serious and obvious errors" in the ruling that generated the appeal.

"You don't need a peg leg or an eye patch," wrote Chief Judge Alex Kozinski in the majority opinion. "When you ram ships; hurl glass containers of acid; drag metal-reinforced ropes in the water to damage propellers and rudders; launch smoke bombs and flares with hooks; and point high-powered lasers at other ships, you are, without a doubt, a pirate, no matter how high-minded you believe your purpose to be."

The opinion is a blow to Sea Shepherd, registered in Oregon and based in Friday Harbor, Wash., known for combating whaling on the high seas. It's a win in a continuing legal battle waged by the plaintiffs, which are led by the Institute of Cetacean Research, a Japanese foundation that cites a scientific-research exception to an international whaling ban.

But the opinion isn't shutting down Sea Shepherd, which continued clashes this week off Antarctica, claiming whalers rammed one of its vessels and deployed stun grenades and a water cannon.

Scott West, a Sea Shepherd spokesman, said that to comply with the appeals court's earlier injunction, the U.S. organization has separated itself from Sea Shepherd Australia, which is now operating the anti-whaling campaign.

"It's not us," West said. "We have separated ourselves from having anything to do with what's going on in Antarctica."

That argument doesn't wash with the Cetacean Institute, which filed suit in 2011 against Sea Shepherd and its president, Paul Watson, whom the judges' opinion describes as eccentric.

Gavin Carter, an institute spokesman in Washington, D.C., noted that Sea Shepherd split its branches after the panel of judges issued a preliminary injunction Dec. 17.

"I would imagine that the courts would be able to see through what they've done," Carter said. "The question now is whether the nations that are flagging the Sea Shepherd vessels, which are the Netherlands and Australia, are going to be willing to continue providing a flag for vessels that the U.S. courts have deemed to be engaging in acts of piracy."

The appeals court judges said their opinion should be taken not as an endorsement of whaling but as a signal that "violent vigilantism by U.S. nationals in international waters" is not condoned.

Judge Jones, a former prosecutor who is a half brother of musician Quincy Jones, has been presiding in Seattle in the case.

The Cetacean Institute appealed Jones' decisions to dismiss its piracy claims and to deny the foundation's request for an injunction against Sea Shepherd. Miller Nash lawyer John Neupert presented arguments in October, supported by his law partner, Chris Helmer.

In the latest opinion, Kozinski wrote that Jones erred in dismissing the piracy claims. "Damaging Cetacean's ships could cause them to sink or become stranded in glacier-filled Antarctic waters," he wrote, "jeopardizing the safety of the crew."

Kozinski wrote in the majority opinion that the case, should be transferred to another district judge, drawn at random, as the case continues in the courts.

But another panel member, Circuit Judge Milan Smith, dissented on that point, saying Jones would be able to put aside his previous views and follow the appeals court's requirements.

"The Sea Shepherds are pirates. Period," Smith wrote. "No district judge could fail to grasp the clarity and firmness of our opinion."